The direct observation of solidification as a function of gravity level
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NASA S p a c e P r o c e s s i n g P r o g r a m is d e v e l o p i n g a p p l i c a t i o n s of s p a c e f l i g h t that w i l l a d v a n c e m a t e r i a l s s c i e n c e a n d t e c h n o l o g y . T h i s o b j e c t i v e i s m e t by o b taining r e s e a r c h r e s u l t s which contribute t o u n d e r standing a n d improving p r o c e s s e s used o n t h e g r o u n d a n d developing m e a n s t o p r o c e s s products o f high v a l u e in s p a c e . O p p o r t u n i t i e s t o p e r f o r m e x p e r i m e n t s in l o w g ( w h e r e g i s d e f i n e d a s 980 c m f s 2) a r e , t h e r e f o r e , e s s e n t i a l f o r p r o g r e s s in s p a c e p r o c e s s i n g , a n d t h e S p a c e P r o c e s s i n g P r o g r a m h a s b e e n a c t i v e in e x p e r i m e n t a t i o n o n all available m a n n e d s p a c e flights. T h e c o n d i t i o n o f n e a r weightlessness o b t a i n e d in s u b o r b i t a l o r o r b i t a l f l i g h t c a n n o t be d u p l i c a t e d o n E a r t h e x c e p t in f r e e - f a l l e x p e r i m e n t s w h e r e t h e m a x i m u m d u r a t i o n is o n t h e o r d e r o f seconds. T h e S p a c e P r o c e s s i n g Rocket Experiment Project continues t o develop m a t e r i a l s s c i e n c e experiments through the r e m a i n d e r of the 1970's. Approximately 5 t o 7 m i n o f a c c e l e r a t i o n l e v e l s o f 10-5 g a r e a v a i l a b l e with t h e B l a c k B r a n t VC Sounding Rocket. This p a p e r will p r e s e n t t h e r e s u l t s o f a c a s t i n g e x p e r i m e n t p e r f o r m e d u n d e r t h e s e low g conditions. T h e transition between the columnar and the e q u i a x e d z o n e in c a s t i n g s h a s b e e n t h e s u b j e c t o f n u m e r o u s p u b l i c a t i o n s t-3 T h e o r i e s d e a l i n g w i t h t h i s p h e n o m e n o n h a v e d e p e n d e d on g r a v i t y a s t h e d r i v e r f o r f l u i d f l o w a n d s u b s e q u e n t c r y s t a l m u l t i p l i c a t i o n .3'4 T h e o n l y t h e o r y w h i c h is a c t i v e in t h e a b s e n c e o f gravity 1 s u g g e s t s t h a t t h e equiaxed z o n e f o r m s as a r e s u l t of constitutional supercooling a h e a d of the interface. Experimenters have attempted to circumvent the g r a v i t y i n f l u e n c e by u s e o f m a g n e t i c f i e l d s , 5 m e t a l s c r e e n s ,6 o r o t h e r i m p e d i m e n t s t o f l u i d f l o w . N o n e of t h e s e t e c h n i q u e s a c t u a l l y e l i m i n a t e s the g r a v i t y f o r c e . Through the S p a c e P r o c e s s i n g Applications R o c k e t ( S P A R ) p r o g r a m , t h e p r e s e n t e x p e r i m e n t was o p e r a t e d a t a g r a v i t y a c c e l e r a t i o n a r o u n d 10-~ g , a MARY H. JOHNSTON and CAROLYN S. GRINER are Materials Engineers, Manufacturing and Processes Laboratory, Marshall Space Flight Center, AL 35812. Manuscript submitted April 2 9 , 1976 METALLURGICAL
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